Vanadium oxide (VOx) films are useful because the electrical resistance of the film is very sensitive to temperature over a useful range of temperatures. This means that relatively small changes in temperature cause easily detected changes in the electrical resistance of the VOx film. In other words, VOx has a high temperature coefficient of resistivity or TCR. This property of VOx films can be used advantageously to build many different kinds of sensors. For example, VOx thin films are used to build microbolometers, the sensors in thermal cameras.
However, VOx films having such desirable properties are difficult to manufacture. Single crystal VO2 material undergoes a phase change from semiconductor (insulator) to metal with a decrease in electrical resistivity of several orders of magnitude as temperature is increased through 67-68° C. as observed by F. J. Morin, Phys. Rev. Lett. 3, 34 (1959). This temperature is too high for many applications and the change in electrical resistivity is too sharp. For example, VOx films used for thermal cameras require TCR characteristics with a much smaller change in resistivity spread out over a wider temperature range to include room temperature (˜20° C.). U.S. Pat. No. 6,127,914 a mixed crystal VNxOy, where 0<x<1 and 2≦y≦13/6, as a material with a high TCR and a low specific resistance. However, this material has not been widely adopted, particularly in sensors, such as bolometers. The manufacture of this mixed crystal material has several drawbacks. For example, the reported deposition rate is low, making it undesirable for commercial manufacture. In addition, the mixed crystal VNxOy is deposited on a heated substrate to 300° C. Heating the substrate to such temperatures is incompatible with manufacturing processes commonly used to manufacture bolometers.
One problem faced in manufacturing VOx films is that both V and O are extremely chemically reactive and, as a result, their reaction is difficult to control. A frequent symptom of loss of control is extreme instability of process parameters related to the O-reactant supply, which results in growth of VOx with unwanted or inconsistent film properties. The chemical reactions that produce the VOx film often sensitively affect the film's properties, such as the structure and composition of the film, the film thickness, the uniformity of thickness, the sheet resistance (resistivity×thickness), and the uniformity of resistivity measured laterally over the film. For example, both the VOx layer deposition rate and its resistivity appear to be sensitive to unintended variations in the deposition process, even when the values of all adjustable process parameters remain the same or are nominally constant. As a practical matter, this sensitivity to unintended variations is so great that, from run to run using conventional processes, layer thickness and resistivity for amorphous VOx films may both deviate from the desired values by ±10%, and deviations of ±20% or even ±50% are not uncommon, all for what were intended to be identical runs. In addition, the layer thickness and resistivity of the VOx film are not uniform across the substrate. U.S. Pat. No. 6,127,914 does not address the problems associated with film uniformity and reproducibility.
The disadvantages of conventional VOx layer deposition significantly increase labor and material costs. Such costs include extended process qualification time, extended chamber conditioning time, more frequent chamber maintenance and reduced product yields. Accordingly, there remains a need for new materials useful in sensors and other applications, and new manufacturing processes and machines for manufacturing thin layers of the materials.